Impacts of Methane on Carbon Dioxide Storage in Brine Formations

In the context of geological carbon sequestration (GCS), carbon dioxide (CO 2 ) is often injected into deep formations saturated with a brine that may contain dissolved light hydrocarbons, such as methane (CH 4 ). In this multicomponent multiphase displacement process, CO 2 competes with CH 4 in terms of dissolution, and CH 4 tends to exsolve from the aqueous into a gaseous phase. Because CH 4 has a lower viscosity than injected CO 2 , CH 4 is swept up into a ‘bank’ of CH 4 ‐rich gas ahead of the CO 2 displacement front. On the one hand, this may provide a useful tracer signal of an approaching CO 2 front. On the other hand, the emergence of gaseous CH 4 is undesirable because it poses a leakage risk of a far more potent greenhouse gas than CO 2 if the cap rock is compromised. Open fractures or faults and wells could result in CH 4 contamination of overlying groundwater aquifers as well as surface emissions. We investigate this process through detailed numerical simulations for a large‐scale GCS pilot project (near Cranfield, Mississippi) for which a rich set of field data is available. An accurate cubic‐plus‐association equation‐of‐state is used to describe the non‐linear phase behavior of multiphase brine‐CH 4 ‐CO 2 mixtures, and breakthrough curves in two observation wells are used to constrain transport processes. Both field data and simulations indeed show the development of an extensive plume of CH 4 ‐rich (up to 90 mol%) gas as a consequence of CO 2 injection, with important implications for the risk assessment of future GCS projects.